1. Field of the Invention:
The present invention relates to glass melters for vitrification of wastes. More particularly, the present invention relates to the design of a melter for stabilization of radioactive and hazardous wastes by encapsulation of the waste in glass.
2. Discussion of Background:
Melters and furnaces have been in existence for many years to manufacture metals and glass and to vitrify waste. Early glass melters included inner walls lined with refractory to insulate the vessel from its molten contents. A refractory is an earthy, ceramic material of low thermal and electrical conductivity capable of withstanding extremely high temperatures.
Early glass melters used direct heat from gas or other combustible fuel burned in the upper portion of the melter (the plenum) directly above the pool of molten material. Later, electric melters were developed that used electrical resistance heat applied to the outside melter wall to heat the glass indirectly or applied directly to the glass by one or more electrode pairs. Direct heating is commonly referred to as Joule heating or Joule-effect heating. Initially, the electrodes were positioned above the melt line of the molten material and heat from the electrode electric arcs or plasma arc torches was absorbed by the molten material.
In other electric melter designs, the electrodes are mounted to the floor or the sides of the melter interior so that the electrodes are submerged in the body of molten material. Alternatively, the electrodes extend vertically into the molten material from the top of the melter or horizontally into the molten material through the melter outside wall and refractory.
Many melter design developments have occurred over the years, including the use of various materials to replace refractories, which are relatively expensive. Even soils and other bulky materials that are naturally insulative are being used to replace refractories in melters. Such materials make the melters too large and heavy for many vitrification applications. Also, electrical current can short circuit from the electrodes through the refractory when the conductivity of the melter contents decreases relative to the conductivity of the refractory or through molten high metal contents that accumulate on the bottom floor of waste glass melters. Also, refractories need to be repaired or replaced periodically, thus increasing expense and downtime. Also, if the melter is radioactive, its repair and maintenance necessitates high personnel exposure and, for extremely contaminated melters, may not be economically feasible.
In the process known as "skull" melting, a layer of non-conducting material, usually of the same type as that being melted, is allowed to form a shell or "skull" on the inside wall of the melter. For metal melting, the skulls are typically made from refractory oxides such as Y.sub.2 O.sub.3, ZrO.sub.2, La.sub.2 O.sub.3, as well as mixed oxides such as LaAlO.sub.3, CaZrO.sub.3.
Glass skulls in glass making am also known. U.S. Pat. No. 5,149,488 describes a method and apparatus for rapidly solidifying a material using a "skull" within a pool of molten material. The skull is of the same material as the material to be solidified, thereby avoiding compositional fluctuations or dissolution of refractory into the molten pool. U.S. Pat. No. 5,028,248 also discloses a skull employed in combination with a rotatable shell. The rotatable shell distributes the heat throughout the molten material and provides for an even melt.
However, prior to the instant invention, there existed no vitrification melter design that provides adequate insulation for the melter vessel without the use of a refractory, while being compact, reliable and requiring little maintenance.